This invention relates to a funnel for use in a CRT (cathode ray tube) of a non beam-index type.
An envelope of a CRT comprises a panel portion for projecting video images, a tubular neck portion with an electron gun arranged therein, and a flare-shaped funnel portion connecting the panel portion and the neck portion. Electron beams emitted from the electron gun excite phosphors arranged on an inner surface of the panel portion to emit light so that the video images are projected on the panel portion. At this time, X-rays bremsstrahlung are produced inside the CRT. Therefore, the envelope of the type is required to have a high X-ray absorbability.
A funnel glass is used for the funnel portion. The funnel glass used for the envelope of the CRT is generally referred to as a CRT funnel glass.
In view of the above, the funnel glass is made of a lead glass which contains 10-30 mass % of PbO high in X-ray absorbability and which has an X-ray absorption coefficient of 40 cmxe2x88x921 or more at 0.6 xc3x85.
Such CRTs are classified broadly into a non beam-index type and a beam-index type. In recent years, following the increase in production amount and size of the CRT of the non beam-index type, there arises a shortage of glass parts. In particular, the funnel glass tends to be broken during an assembling process of the CRT. Therefore, the shortage is serious for the funnel glass as compared with other parts and the increase in production amount is in a strong demand.
In order to increase the production amount, an additional melting furnace must be newly installed. However, such new installation of the additional melting furnace results in a high cost and is not preferable.
In view of the above, it is proposed to increase a glass pull rate in an existing melting furnace without newly installing the additional melting furnace. However, if the glass pull rate is increased, a residence time of a glass within the melting furnace is shortened so that the glass is formed into a glass product without being subjected to sufficient fining within the melting furnace. In this event, the number of seeds in the glass product is increased and a product yield or a production efficiency is degraded.
In order to increase the glass pull rate in the melting furnace and to reduce the number of seeds in the glass product, the content of Sb2O3 as a fining agent for a CRT glass must be increased. However, since Sb2O3 is a high environmental load substance, a greater content than that used at present is unfavorable.
In view of the above, proposal is made of a technique of reducing the number of seeds in the glass by addition of CeO2 to Sb2O3. For example, Japanese Unexamined Patent Publication No. 8-31342 (JP 8-31342 A) discloses a CRT funnel made of a lead glass containing Sb2O3 and CeO2. However, the funnel disclosed therein is for use in a beam-index type CRT. In the CRT of this type, coloring of a funnel glass by X rays produced in the CRT results in reduction in indexing beams detected by a photodetector device so that the performance of the CRT is deteriorated. The addition of CeO2 in the above-mentioned publication aims to solve such problem due to the X-ray coloring inherent to the beam-index type CRT funnel. Thus, the glass disclosed therein contains a large amount of CeO2. In addition, the CRT funnel of the beam-index type is a small-sized model not greater than 6 inches and is very small in quantity of production. Therefore, the glass pull rate is small so that the residence time of the glass in the melting furnace is extended. As a result, irrespective of the amount of the fining agent, sufficient fining is performed. In this connection, the above-mentioned publication does not suggest an appropriate type and an appropriate type of the fining agent in case where the glass pull rate is increased.
It is therefore an object of this invention to provide a CRT funnel of a non beam-index type having an excellent seed free grade by enabling the increase in glass pull rate in a production process without requiring the increase in content of Sb2O3.
Other objects of the present invention will become clear as the description proceeds.
According to an aspect of this invention, there is provided a CRT funnel of a non beam-index type, made of a lead glass which contains 10-30 mass % of PbO, which has an X-ray absorption coefficient of 40 cmxe2x88x921 or more at 0.6 xc3x85, and which contains Sb2O3 and an additive comprising at least one of CeO2 and SnO2.
CeO2 and SnO2 have following characteristics. In CeO2, change in valence causes a reaction represented by 2CeO2xe2x86x92Ce2O3+xc2xdO2 to gradually produce an oxygen gas in a temperature range from about 1000xc2x0 C. to about 1500xc2x0 C. In SnO2, change in valence number causes a reaction represented by SnO2xe2x86x92SnO+xc2xdO2 in a temperature range higher than the above-mentioned temperature range in CeO2 to produce an oxygen gas in a temperature range from about 1400xc2x0 C. to about 1600xc2x0 C. Therefore, by the use of Sb2O3 and at least one of CeO2 and SnO2 in combination as a fining agent, a fining gas is increased in a wide temperature range. As a consequence, even if a residence time of the glass in a melting furnace is short, a high fineness is kept.
PbO is a component which improves the X-ray absorption coefficient of the glass. However, if the content of PbO is less than 10 mass %, the X-ray absorbability is insufficient. On the other hand, if the content is greater than 30 mass %, the viscosity of the glass is excessively low so that formation is difficult. Preferably, the content of PbO falls within a range of 15-27 mass %.
Sb2O3 is essential as a fining agent. However, if the content is greater than 2 mass %, the glass tends to be devitrified. Preferably, the content of Sb2O3 falls within a range of 0.01-1 mass %.
CeO2 is a component required to compensate the shortage of the fining gas in case where the glass pull rate is increased. However, if the content is less than 0.01 mass %, a sufficient effect can not be obtained. On the other hand, the content of 0.5 mass % or more brings about no substantial change in fineness and does not provide the effect corresponding to the increase in raw material cost but the number of seeds may be increased to the contrary. In case where the above-mentioned additive comprises CeO2 alone, the content of CeO2 preferably falls within a range of 0.01-0.45 mass %.
SnO2 is also a component required to compensate the shortage of the fining gas in case where the glass pull rate is increased. If the content is less than 0.001 mass %, a sufficient effect can not be obtained. On the other hand, if the content is greater than 2 mass %, the glass tends to be devitrified. In case where the above-mentioned additive comprises SnO2 alone, the content of SnO2 preferably falls within a range of 0.001-1.5 mass %.
In case where the above-mentioned additive comprises CeO2 and SnO2, it is preferable that the content of CeO2 is not smaller than 0.01 mass % and is smaller than 0.5 mass % and that the content of SnO2 falls within a range of 0.001-2 mass %.
In addition, the lead glass may further contain, in mass %, 48-58% SiO2, 0.5-6% Al2O3, 0-5% MgO, 0-6% CaO, 0-9% SrO, 0-9% BaO, 3-9% Na2O, 4-11% K2O, 0-5% ZnO, and 0-2% ZrO2.
SiO2 is a component serving as a network former of the glass. If the content is less than 48 mass %, the viscosity of the glass is excessively low so that the formation becomes difficult. If the content is more than 58 mass %, the coefficient of thermal expansion of the glass is excessively low and fails to match the coefficient of thermal expansion of a neck glass. Preferably, the content of SiO2 falls within the range of 49-57 mass %.
Al2O3 is also a component serving as a network former of the glass. If the content is smaller than 0.5 mass %, the viscosity of the glass is excessively low so that the formation becomes difficult. If the content is greater than 6 mass %, the coefficient of thermal expansion of the glass is excessively low and fails to match the coefficient of thermal expansion of a neck glass. Preferably, the content of Al2O3 falls within a range of 1-5 mass %.
MgO is a component serving to facilitate melting of the glass and to adjust the coefficient of thermal expansion and the viscosity. If the content is greater than 5 mass %, the glass tends to be devitrified so that the formation becomes difficult. Preferably, the content of MgO is 4 mass % or less
CaO, like MgO, is a component serving to facilitate melting of the glass and to adjust the coefficient of thermal expansion and the viscosity. If the content is greater than 6 mass %, the glass tends to be devitrified so that the formation becomes difficult. Preferably, the content of CaO falls within a range of 1-5 mass %.
Each of SrO and BaO is a component serving to facilitate melting of the glass, to adjust the coefficient of thermal expansion and the viscosity, and to improve the X-ray absorbability. If the content of each of SrO and BaO is greater than 9 mass %, the glass tends to be devitrified so that the formation becomes difficult. Preferably, the content of each of SrO and BaO is 7 mass % or less.
Na2O is a component to adjust the coefficient of thermal expansion and the viscosity. If the content is less than 3 mass %, the coefficient of thermal expansion is excessively low and fails to match the coefficient of thermal expansion of the neck glass. If the content is more than 9 mass %, the viscosity is excessively low so that the formation is difficult. Preferably, the content of Na2O falls within the range of 4-8 mass %.
K2O, like Na2O, is a component to adjust the coefficient of thermal expansion and the viscosity. If the content is smaller than 4 mass %, the coefficient of thermal expansion is excessively low and fails to match the coefficient of thermal expansion of the neck glass. If the content is greater than 11 mass %, the viscosity is excessively low so that the formation is difficult. Preferably, the content of K2O falls within the range of 5-10 mass %
ZnO is a component to improve the X-ray absorption coefficient of the glass and to suppress alkali elution. If the content is greater than 5 mass %, generation of devitrifying stones is increased due to volatilization and agglomeration of ZnO. Preferably, the content of ZnO is 4 mass % or less.
ZrO2 is a component to improve the X-ray absorption coefficient of the glass. If the content is greater than 2 mass %, the glass tends to be devitrified so that the formation becomes difficult. Preferably, the content of ZrO2 falls within the range of 0-1.5 mass %.
In addition to the above-mentioned components, other components such as P2O5 and B2O3 may be added up to 1 mass % as far as the characteristics of the glass are not degraded.